In the drilling and completion of oil and gas wells, it has been well known for a considerable period of time to use small amounts of cement slurry pumped into the well to shut off undesired ingress or egress of fluids to or from the formations encountered. For example, it is well known to "spot" a relatively small volume (typically, at most a few barrels) of slurry at the bottom of the well in order to shut off ingress of bottom water, or the like. It is also well known to define a vertically limited zone (by bridge plugs, packers, or the like) after which again a relatively small volume of cement slurry is pumped into this defined zone and pressure applied to "squeeze" the slurry into an opening in a casing or perforation in the casing, or into a relatively limited open-hole zone. Since application of considerable compressional stress to cement slurry causes very little change in its overall volume, the disappearance of the cement slurry is ordinarily ascribed to fracturing the adjacent formation and causing the cement to fill the interstices of this fracture and any other openings or voids that might exist, such as defects in the cement behind the casing (which is supposed to have completely filled the annulus during the casing cementing job). Occasionally, it is found that the cement will leave the limited zone under effectively the pressure in the adjacent formation. After all, the viscosity of a cement slurry is quite low, being of the order of a few centipoises at most, so it tends to flow into the formation rather than stay in the wellbore, if there is an opening through which it can flow. The water in the slurry tends to leak away and leave some of the finely ground cement in a much more concentrated slurry.
In young geological formations, particularly those of a relatively unconsolidated type, such as the shallower Gulf Coast formations in the United States, there are two difficulties in using cement plugs or cement squeeze jobs. One has already been mentioned: the tendency of the relatively low viscosity cement slurry to flow into the formation. This is particularly disadvantageous if the cement is being used to seal immediately around the edge of the bore. The operator wants to keep the slurry from further penetration radially outward. This tends to minimize the likelihood of the cement doing an adequate sealing job right at the point where such seal is essential.
Also, after the slurry has hardened and the cement is no longer in the liquid phase, it is harder than the adjacent formation. Whether the cement was spotted on bottom or used as a plug in the bore, it is likely when the usual practice of redrilling is carried out, that the drill bit will be deflected by the relatively harder cement plug and tend to sidetrack to form a bore bypassing the region for which the seal was intended, and causing repeated difficulty.
A widely used remedy for solving lost-circulation problems in the relatively soft, young formations along the Gulf Coast and similar areas around the world is to use a different material for the squeeze job. The basic ingredient is diatomaceous earth. As I use the term, "diatomaceous mix" refers to a mixture of powdered diatomaceous earth containing 2 wt. percent of ground lime and 8 wt. percent ground asbestos fibers. A typical slurry for such squeeze consists of the following materials in approximately the portions indicated:
25 lbs/bbl of diatomaceous mix, PA1 0.6 bbl/water, PA1 3 lbs/bbl of a lost-circulation material consisting of flakes, flax, fibers, and granules of medium size, and an equal amount of the same in the coarse size, PA1 20 lbs/bbl of a medium size of particulate strong nut hulls, PA1 5 lbs/bbl of coarse Phenoseal (phenolic resin laminate), and sufficiently finely ground barite to weight the slurry as desired, for example, to the order of 17 lbs/gal.
The problem experienced when using this slurry for squeeze jobs is that its compressive strength is quite low, of the order of 0 to 40 psi. Accordingly, subsequent drilling, rotating, and tripping of the bit (that is, removal of a worn bit and installation of a new bit) weakens the squeeze and once again mud returns are lost to the formation. In one small region along the Louisiana Gulf Coast near Lake Charles, for example, during the four years 1970-1973, 13 squeezes using this kind of slurry were performed and only four lasted longer than a week. As much as $60,000 could have been saved on one of these wells alone if the seal had held.
As is described later more specifically, I decided to reduce the amount of diatomaceous mix in the material drastically, to the order of five to ten pounds per barrel, and add ordinary oil well-type cement in amounts ranging from 5% to 50% by weight of the total slurry mix. This forms the basis of this invention.
The problem of what to do about solving open-hole lost circulation in such relatively soft and unconsolidated formations had existed for quite a number of years. Attempts to use a diatomaceous mix squeeze, while frequently unsuccessful, have been considered, especially in Louisiana, the best effort made to improve on the situation.
Since making this invention, I have been made aware of the patents listed below concerning mixtures of Portland cement and diatomaceous earth used in slurry form to make a cement. The cements described in these patents in every case are for use in construction or in the cementing of casing in oil wells. There is no teaching in any of them about the use of mixtures of these materials, in conjunction with lost-circulation agents, for the purpose of sealing off lost-circulation zones and the like. In fact, the general understanding among petroleum engineers as to the use of squeeze cementing techniques was that the addition of cement to the diatomaceous mix squeezes employed would be quite detrimental, since the main purpose for using the diatomaceous mix was to have a material which was mechanically considerably softer than the cement itself. These patents are as follows:
U.S. Pat. No. 2,585,336, Bollaert, et al., in which a mixture is made using from 2% to 100% diatomaceous earth, compared to the content of the cement in the slurry. The aim of the inventors was to prevent perlite from settling and to produce a lightweight cement. The diatomaceous earth-cement described in the disclosure is a mixture of Portland cement, perlite and diatomaceous earth, lime, and asbestos fibers.
U.S. Pat. No. 2,793,957, Mangold, et al., refers to a highly permeable cement formed by use of the same basic mixtures of diatomaceous earth with Portland cement, the diatomaceous earth present being from five to seven times the proportion of the Portland cement in the slurry. The aim of the inventors was to produce a light highly permeable cement, entirely opposite to the purpose of my invention.
U.S. Pat. No. 2,961,044, Shell, discusses and claims a cement composition which has diatomaceous earth in the amounts of from 30% to 70% of the Portland cement. The reason for using the diatomaceous earth was to prevent the strength retrogression of a salt-saturated cement. Thus, while Shell wishes (among other uses) to employ his mixture for squeeze cementing, he produces a relatively high-strength cement plug. There is a real tendency when redrilling such a plug for the bit to be deflected or sidetracked so that the new hole is beside rather than through the bore and the seal is ineffective. This is completely different from my invention which minimizes such tendency by producing a plug at least as drillable as the formation in which it is set. Also, Shell is directed to operations using salt-saturated cement slurries, while I prefer using a fresh or brackish water slurry. I employ lost-circulation agents; he makes no teaching of using such additives. Accordingly, his teaching is quite far from mine.
Two further U.S. Pat. Nos. 3,467,198 and 3,558,335, Messenger, describe cement compositions having diatomaceous mix present in the amounts from 0.5% to 10% of the amount of Portland cement present to prevent solids-settling.